Security systems use many detection devices for monitoring an area. These devices include motion detectors, window contacts, glass break detectors, video cameras, temperature sensors, smoke detectors, etc. These devices send signals to an alarm panel, which accepts the signals, processes them, and generates alarm messages to a central alarm station if necessary. The detectors may be hardwired to the alarm panel or may contain an RF transmitter that sends the signal wirelessly to the alarm panel. Detectors with wireless transmitters are commonly used because of their installation ease. These wireless detectors use RF transmitters in the 260 MHz to 470 MHz range and send the signals to the alarm panel multiple times to ensure receipt of the signal. In the United States, the transmitted signals are governed by FCC regulations as explained below. Governmental agencies in other countries may also have similar regulations.
Wireless transmitters for security systems may be one of two basic types, single frequency and frequency hopping. The single frequency transmitters send all of the alarm signals to the alarm panel using a selected carrier frequency. The receiver in the alarm panel decodes only signals at the selected frequency of the carrier signal. The single frequency transmitters are regulated by Part 15 of the FCC regulations, which requires a signal to be less than [41.66 (μV/m)/MHz]*F-7083.33 μV/m, where F is the frequency of the transmitted signal. It also requires that a signal be present for less than 5 seconds.
The frequency hopping transmitters transmit signals over fifty different frequency channels. The transmitters use an algorithm to determine the frequency at which each alarm signal will be transmitted. The receiver in the alarm panel senses signals over a wide frequency range and also has a corresponding algorithm to determine which frequency band to locate the alarm signal. The electronics for both the transmitter and the receiver are much more complex and costly than the single frequency systems. In addition the frequency hopping transmitters are regulated by part 15.247 of the FCC regulation which requires the signal to be less than 50 mV/m.
The single frequency systems have a number of benefits such as a more robust transmitted signal, less complex hardware, and lower cost, however, can be susceptible to in-band interference. Interference occurs when, for example, a signal is radiated by an unintentional radiator in the vicinity of the alarm system receiver in the same frequency band as the alarm system receiver.
The frequency hopping system transmits at many frequencies and is less susceptible to in band interference because, even though part of the alarm signal may be transmitted at the frequency of the interference signal, it will transmit the signal again at a different frequency. If the receiver is tuned to the same frequency as the interference signal at first, it will receive only the interference data, but once it is tuned to a different frequency (or frequencies) it will be able to decipher the transmitted alarm signal. It is desirable to create a system that has the benefits of a frequency hopping system with the cost and simplicity of a single frequency system.
It is therefore an object of the present invention to provide an alarm system receiver that is capable of receiving alarm signals from a detection device when an interference transient is present.
It is a further object of the present invention to provide an alarm system receiver that is capable of sensing signals at more than one frequency.
It is a further object of the present invention to be able to detect an interference transient at one frequency and automatically switch to receiving signals at a different frequency.
It is a further object of the present invention to provide an alarm system receiver that is less complex and costly than a frequency hopping system receiver.